Composition and method for surface-sizing paper and the like

ABSTRACT

Paper and like webs are given improved surface properties and resistance to ink penetration by means of a composition, readily dispersible in water at about 65*-70* C., of a small proportion of higher ketene dimer with a highly water soluble dextrin phosphate having 0.3-3 percent of phosphorus molecularly bound therein and typically containing 1-3 percent of free phosphate, or with a mixture of a water soluble noncationic starch derivative of low viscosity and suitable alkali metal phosphate. A composition particularly valuable for sizing papers of highalum content contains the dextrin phosphate and 4-8 percent of ketene dimer with up to 20 percent of guar flour, about 2-5 percent of sodium fluoride and about 3-5 percent of sodium carbonate.

United States Patent Schobinger et al. [4 Jan. 25, 1972 [54] COMPOSITION AND METHOD FOR 2,776,226 1/1957 Hart ..1 17/64 SURFACESIZING PAPER AND THE 2,884,413 4/1959 Kerr ...106/210 LIKE 3,276,359 10/1966 Worthen eta1.... ...117/156 3,245,816 4/1966 Schwalbe ...1 17/156 [72] Inventors: Ulrich Schobinger, Zug; Cla Christoffel, 35241796 8/1970 Yul mum/2m waedenswil; Kurt Berner, pfaeffikon all Weisgerber of Switzerland Primary Examiner-Theodore Morris [73] Asslgnee: g waedenswll Swltzel' Attorney-Albert C. Johnston and Robert E. lsner 22 Filed: May 21,1969 [571 ABSTRACT [21] Appl No.: 826,658 Paper and like webs are given improved surface properties and resistance to ink penetration by means of a composition, I readily dispersible in water at about 65-70 C., of a small pro- [30] Foreign Application Priority Data portion of higher ketene dimer with a highly water soluble dextrin hos hate havin 0.3-3 ercent of hos hor s 2 6 P P P P P P May 1 l9 8 Swnzefland 7533/68 molecularly bound therein and typically containing 1-3 per- [52] Us. CL 106/213 117/156 cent of free phosphate, or with a mixture of a water soluble 51] Inn CL C68]! 25/02 noncationic starch derivative of low viscosity and suitable al- I 58] Field i 162/175 kali metal phosphate. A composition particularly valuable for 1 7 1 172156 62 sizing papers of high-alum content contains the dextrin phosphate and 4-8 percent of ketene dimer with up to 20 per- [56] References Cited cent of guar flour, about 2-5 percent of sodium fluoride and about 3-5 percent of sodium carbonate. UNITED STATES PATENTS 17 Claims No Drawings 2,762,270 9/1956 Keim .,..ll7/ l56 COMPOSITION AND METHOD FOR SURFACE-SIZING PAPER AND THE LIKE This invention relates to a composition and method for sizing surfaces of paper, cardboard or like webs of matted fibers, and particularly surfaces of such webs which have high contents of wood pulp or of alum, so as to improve their properties and especially to make them resistant to penetration by printing ink. The invention also relates to the resulting sized paper or like fibrous web material.

Dilute emulsions of higher ketene dimers dispersed in aqueous solutions of thickening agents, such as cationic starch derivates, have been proposed heretofore for the internal (beater) sizing and even for some surface sizing of paper. See U.S. Pats. Nos. 2,627,477, 2,762,270, 3,070,452 and 3,l30,l 18. These known emulsions ordinarily are preformed and shipped to users, since the ketene dimers would be decomposed at the temperatures above 90 C. which are required for efficient dispersion of the starch derivatives used with them. The preformed emulsions, however, have a very limited stability, are costly to ship due to their high water content, and are either unsuitable or too costly in the quantity required for the effective sizing of papers made from wood pulp or which have an alum content of more than about 0.7 percent by dry weight.

It has been found that these disadvantages can be overcome and that valuable improvements of the surface properties of paper and like matted fibrous materials can be attained by means of a composition of a relatively small proportion of one or more of the known higher ketene dimers with a major proportion of a highly water soluble thermally degraded phosphated starch having a low viscosity in solution, classifiable as a dextrin phosphate, of the nature set forth in our copending U.S. application Ser. No. 826,707, filed May 21, 1969.

it has also been found that benefits in respect of ink holdout, similar to those obtained from such a composition, can be attained by using with the ketene dimer, in place of the dextrin phosphate, a mixture of a water-soluble noncationic (i.e., anionic or nonionic) starch derivative of low viscosity in solution and an alkali metal phosphate, or mixture of alkali metal phosphates, having a pH near neutrality, such, for example, as a di-alkalimetal phosphate, a pyrophosphate such as Na i-IP 0, or a mixture of primary and secondary orthophosphates.

The ketene dimers particularly suitable for the sizing composition of the invention have the general formula [RCH=C=]gr in which R is an alkyl radical having at least eight carbon atoms, a cycloalkyl radical having at least six carbon atoms, or a phenyl, naphthyl or benzyl radical.

The dextrin phosphate found effective for the purposes of the invention is a thermally degraded phosphated starch of relatively low molecular weight which has very high cold water solubility and a relatively stable low viscosity in water solution, together with an extremely light color, excellent film forming properties and compatibility with a wide variety of organic substances. This starch product typically contains between 0.3 and 3 percent by weight of phosphorus molecularly bound therein and has a cold (25 C.) water solubility of from well above 30 percent to complete (100 percent) solubility, with a solubility of 75 to 100 percent in water at 65 C. and a viscosity preferably as little as 5 to 30 c.p.s., and in all cases below 500 c.p.s., when placed in 5 percent solution in water at 25 C. It also ordinarily contains a small amount of free phosphate, corresponding to about 1 to 3 percent by weight of phosphorous supplied in alkali metal phosphate and/or phosphoric acid not molecularly bound in the degraded phosphated starch substance. The dextrin phosphate can be prepared by the method set forth in said copending application, of which a typical example is given hereinbelow.

When a mixture of a water soluble noncationic starch derivative of low viscosity and an alkali metal phosphate is to be employed instead of the dextrin phosphate, this mixture may be prepared as by mixing such a starch derivative with, for example, about 4 to 6 percent, based on its dry weight, of disodium phosphate. The noncationic starch derivative may be a white dextrin, oxidized starch, a product of the partial hydrolysis of starch by enzymes, a starch ester or a starch ether, provided that it has a water solubility of at least percent at 65 C. and a viscosity less than c.p.s., preferably of about 5 to l0 c.p.s., in 5 percent solution in water at 25 C.

Compositions especially suitable for the surface sizing of paper or cardboard so as to make good use of the ink holdout property of the ketene dimers are provided in the form of mixtures containing by dry weight at least about 55 percent and preferably at least 70 percent, such as 75 to 80 percent or more, of the dextrin phosphate, or of the above-mentioned mixture of water soluble white dextrin and alkali metal phosphate, with about 2 to 15 percent, preferably about 4 to 8 percent, of one or more of the known ketene dimers.

The surface-sizing compositions here provided are particularly advantageous in that they can be prepared and shipped in a ready-for-use dry pulverulent form which is highly stable and which, due to their high water solubility at 65 C., can easily be dispersed thoroughly in water at temperatures between 50 and 80 C., including the temperatures of about 65 to 70 C. which generally are required for emulsification of the ketene dimers. Thus, the sizing liquid can be prepared readily as and when desired for use, at temperatures below those (above about 80 C.) at which the ketene dimers are susceptible to decomposition.

The sizing liquids so prepared from compositions containing the dextrin phosphate with the ketene dimer give excellent surface sizing effects when made with a dry substance content of between 2 and 8 percent by weight. In addition to their important advantage of being easily prepared by the user, they enable increases of the speed of surface sizing work, as compared with that attained in the use of known ketene dimer emulsions. They also give improvements of the surface properties of the sized sheet, as determined by the well-known Dennison wax-pick test for rupture strength [described in Handbook on the Manufacture of Cardboard," Vol. II, p. 2014 1963 edition of Dr. Sandig Verlag, Wiesbaden)].

The ketene dimers contained in the new surface sizing compositions are ordinarily supplied commercially in admixture with a nonionic emulsifier, the content of which amounts, for example, to between 10 and 30 percent and in particular about 15 to 20 percent of the weight of the ketene dimer. Accordingly, the present surface sizing compositions will ordinarily contain a nonionic emulsifier in the stated amount; and if none should be present with the ketene dimer used, a nonionic emulsifier in such amount is preferably added to aid in the dispersion of the dimer.

Since the dextrin phosphate is itself ionogenic, and since it ordinarily contains a small amount of free phosphate, corresponding to about 1 to 3 percent by weight of phosphorus supplied in alkali metal phosphate and/or phosphoric acid not molecularly bound by reaction in the degraded phosphated starch substance, the dextrin phosphate in the sizing compositions itself serves to counteract the tendency of alum to render the ketene dimer ineffectual. So also does the alkalimetal phosphate content of the mixture that may be used in place of the dextrin phosphate,

For the surface sizing of paper or pulp sheets having an alum [Al (SO '10H 0] content above about 0.7 percent by dry weight, which might interfere with the sizing if not fully counteracted, the compositions may be provided advantageously with between 0.5 and 10 percent by dry weight, preferably about 1.5 to 5 percent, of additional substance that sequesters, or forms a complex with, aluminum ions. This additional substance is preferably one or a mixture of compounds such as alkali metal fluorides, nitrilo-tri-acetic acid, ethylenediaminetetra-acetic acid, tartaric acid, citric acid, phosphoric acid, and water soluble salts of these acids. Compositions having excellent properties for sizing papers of high alum content are obtained when, in addition to the ketene dimer and the dextrin phosphate or above mixture, about 2 to percent by dry weight of sodium fluoride is included in the composition.

Ifthe sheet to be sized also has a high content of wood pulp, the compositions may be provided advantageously with between 2 and percent by dry weight, preferably about 3-5 percent, of an alkali metal carbonate, preferably sodium carbonate.

The paper sizing compositions may also be provided advantageously with up to percent, preferably about 5 to 10 percent, by dry weight of a water soluble polysaccharide that forms highly viscous solutions, such, for example, as carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, guar flour, or an alginate. These are cold water soluble anionic or nonionic polysaccharides. Their use with the dextrin phosphates enables a very considerable reduction, of up to as much as about 50 percent, of the amount of the relatively expensive ketene dimer that is required for a given sizing effect when known emulsions of the ketene dimers are used, particularly for sizing cellulose paper or paper having a high content of wood pulp.

The invention is further illustrated by the following examples of preferred embodiments.

TYPICAL PREPARATION OF THE DEXTRIN PHOSPHATE To 100 kg. of commercial corn starch was added by spraying a solution made up from 20 l. of water, 10 kg. of tetrasodium pyrophasphate (Na,P 0,-l0l-I 0) and 6.2 kg. of 85 percent orthophosphoric acid, corresponding to 3 parts of phosphorus per 100 parts of starch, of which 54 percent was in the orthophosphoric acid. The pH of the solution was 3. The finely divided mixture of starch and phosphorus compounds, with stirring, was heated for 3% hours under a vacuum of 560 mm. Hg to 85 C. The mixture was then heated to 140 C. within 1 hour under a vacuum of 70 mm. Hg, practically all the water being thus removed, and held for 1% hours at 140 C. under the same vacuum. The resulting dextrin phosphate was then cooled to room temperature under a vacuum of 50 to 70 mm. Hg.

This product dissolved rapidly in cold water and showed a viscosity of 5 c.p.s. at C. in 5 percent solution. Its solubility in water after 1 hour was 80 percent at 25 C. and 99 percent at 65 C. After washing with an 80:20 mixture of alcohol and water the product contained 1.10 percent of phosphorus con sidered to be molecularly bound.

The dextrin phosphate so produced is taken directly from the reaction vessel, or vacuum cooker, in a finely divided, white condition suitable for industrial users. Because of the limited amount of added phosphate and of the extremely light color, no washing or purifying treatment or further processing of the product is required.

The phosphorus compounds suitable for the reaction include any of the alkali metal orthophosphates, hypophosphates, metaphosphates, pyrophosphates and polyphosphates, including polymetaphosphates, in admixture with any of the ortho-, pyro-, hypoand meta-phosphoric acids.

The starch to be converted may be any of various naturally occurring starches, such as corn starch, potato starch, wheat starch, waxy maize, tapioca starch, milo starch, rice starch, or others.

SURFACE SIZING PAPER AND THE LIKE EXAMPLE A Three sizing liquids were prepared at 4 percent concentration of dry substance from mixtures, respectively, of (l) a dextrin phosphate prepared as above specified, (2) a commercial oxidized corn starch and (3) a commercial thin-boiling potato starch, with 5.3 percent of hexadecylketene dimer in each case, based on the dry weight of the starch product. The

dextrin phosphate was dispersed in heated water with the ketene dimer. Products (2) and (3) were boiled separately and after cooling emulsified with the ketene dimer. The three liquids were applied as surface sizings to cellulose paper weighing 1 l0 g./m. and the sized paper samples were tested for amount of size pickup, breaking length, elongation, and Dennison wax-pick value. The test results are shown in Table l below.

TABLE I Breaking Dennison Size Length, Elon- Wax-pick Paper Pickup Meters gation Value Cellulose paper, 3700 m. 3.8% 6/7 untreated Same paper surface I sized with: 1.95% 4200 m. 3.6% 13 14 1. Dextrin phosphate ketene dimer 2. Oxidized corn 2.0% 4700 m. 3.5% 10/1 1 starch ketene dimer 3. Thin-boiling 2.1% 4600 m. 3.5% 9/10 starch ketene dimer As the table shows, the composition containing the dextrin phosphate with the ketene dimer gave the paper better surface properties, especially as to Dennison wax-pick test values, than did the composition of either of the other starch products with the same amount of the ketene dimer.

EXAMPLE B A dry pulverulent mixture of the low molecular dextrin phosphate with a hexadecylketene dimer and additives was prepared to contain:

A sizing liquid was prepared by sprinkling and stirring g. of the above mixture into I l. of water at 65 C. After 10 minutes, aliquot parts of the stock liquid were diluted with water to dry substance concentrations of 0.5, l, 2 and 3 percent, respectively. The emulsions thus obtained were applied for the surface sizing of (l) a wood-free, voluminous, bleached cellulose paper and (2) a paper made of pure wood pulp. Each paper had a weight of 100 g. per m The sizings were applied by a laboratory sizing press at a feed velocity of 0.5 meter per minute, with a squeeze-out pressure of 30 kg. per cm.

The papers thus surface sized were tested for ink holdout properties in comparison with similar papers sized similarly by the use of 0.5, l, 2, 3 and 4 percent emulsions of the following substances:

Polyoxyethylene-sorbitolhexaoleate Total 100.0

The preparation of these comparison emulsions was necessarily more difficult. The starch product ingredients were first dissolved by boiling the stated parts of them, in grams, in 1 1. of water. Then the solution was cooled to 65 C. Then the emulsifier and the ketene dimer were added and stirred in for 15 minutes, and aliquot parts of the resulting stock liquid were diluted with water to give the sizing emulsions.

1n the ink holder tests, ink patches were applied in the same way to the sized paper samples and the time required for the applied ink to cease floating on, i.e., to penetrate, the sized surface was determined for each sample. The cellulose paper sized with the 1 percent emulsion of mixture 1 showed an ink holdout time exceeding that shown by like paper sized with the 1.5 percent emulsion of mixture 2, while the 1 percent emulsion of the latter gave no appreciable holdout effect. The wood pulp paper sized with the 2 percent emulsion of mixture 1 showed an ink holdout time of 10 minutes and more, whereas the wood pulp paper sized with emulsions of mixture 2 required the 3 percent emulsion to attain a holdout time of about 2 minutes and required the 4 percent emulsion to approximate the holdout time of the 2 percent emulsion of mixture i.

As shown by these tests, a given surface sizing effect can be attained by the use of a composition according to the invention with the use of an amount of the ketene dimer about 50 percent less than that required when using a composition employing cationic starch.

Substantially the same results as those given by mixture 1 can be attained, under like conditions in other respects, when the hexadccylketene dimer is replaced by a ketene dimer derived from any of various naturally occurring saturated and/or unsaturated fatty acids or fatty acid mixtures.

In order to determine the stability of the above-described emulsions in the presence of alum such as that which may get into the sizing liquid from the paper sheet in a sizing press, emulsions prepared from mixture 1 and mixture 2 were each adjusted to pH 7.2 with soda and mixed with quantities of alum [A1 (SO.,) -18l-l progressively increasing from 1 g. to 5 g. thereof per liter of the size. Paper samples surface sized with the alum-treated emulsions were then tested for ink holdout properties.

In these tests, the ink holdout time of paper sized with the emulsion of mixture 2 dropped precipitously when the emulsion had an alum content of about 1 percent whereas an alum content of about 3 percent was required in order to bring about a comparable drop of the ink holdout time of paper sized with the emulsion ofmixture 1.

In another series of alum stability tests, samples of a cellulose paper were presoaked in solutions of progressively increasing amounts of alum in water, which ranged in pH from 4.0 down to 3.30. These samples were then surface sized with the 2 percent emulsions of mixture 1 and mixture 2 and tested for ink holdout properties.

The ink holdout time (about 16% minutes) of the alumtreated paper sized with the 2 percent emulsion of mixture 2 dropped when the presoaking pH went below 4.0 and was entirely lost at a pH below 3.60 corresponding to an alum content of about 0.75 percent in the paper. On the other hand, the paper sized with the emulsion of mixture 1 retained an ink holdout time of about 20 minutes until its presoaking pH was reduced to 3.40, corresponding to an alum content of about 1.5 percent in the paper; its holdout property was entirely lost only at a pH below 3.35 corresponding to an alum content of about 2.2 percent or more in the paper.

EXAMPLE C A dry pulverulent mixture was prepared with the following composition:

Mixture 3 Parts by weight Dextrin phosphate Mixed tetradecyland 8 hexadecyl-ketene dimers Sodium carbonate 5 Sodium fluoride 3 Total 126 This mixture was stirred in water at 65 C. for 15 minutes to provide a ready-for-use emulsion having a dry substance concentration of about 5 percent. This emulsion was tested for surface sizing effects in comparison with a commercially available surface sizing composition identified as Basoplast 280D, prepared from a cationic acrylate dispersion and boiled oxidized starch. Comparative test results are shown in table 11 below.

tion after l0 minutes As will be evident from the table, the composition making use of the invention gave sizing effects which were considerably better in all respects than those given by the commercial available composition used for comparison, yet a much smaller quantity of the former is consumed.

EXAMPLE D A dry pulverulent mixture was prepared with the following composition:

Mixture 4 Parts by weight White dextrin" 96 Guar flour 10 Mixed tetradecyland 8 hexadecyl-ketene dimers Sodium carbonate 5 Sodium fluoride 3 Disodium phosphate 4 (Na,H P0,'2H,0)

Total 12s The white dextrin was an acid (HCl) degraded corn starch having a viscosity of20 c.p.s. in 5% solution in water at 25 C. and a solubility of 97% in water at 65 C.

The same paper as in table 11 was treated with a 5 percent solution of mixture 4, and the sized paper showed the properties given in table Ill below, in which the data are comparable with those shown in table 11.

TABLE 111 Paper Sized with Mixture 4 Weight in g.lm. 98 Ash content 15% Size Absorption in g.lm. 2.8 Dennison test value 4/5 B rsting pressure 1.5 kgJcm Breaking length 2610 m after minutes As will be evident from these test data, the degree of ink penetration is practically the same as that attained by use of the dextrin phosphate, but the Dennison value and the physi- 1 cal properties are considerably lower.

What is claimed is:

l. A composition for surface-sizing paper, cardboard or a like web of matted fibers, comprising a finely divided admixture of a minor proportion of a higher ketene dimer with a major proportion of a cold-water-soluble dextrin phosphate consisting essentially of a thermally degraded phosphorylated starch having between 0.3' and 3 percent by weight of phosphorus molecularly bound therein and having a light color, a solubility higher than 30 percent in water at 25 C. and a viscosity in the range of 5 to 500 c.p.s. when in 5 percent solution in water at 25 C., or with a major proportion of a mixture of a water soluble noncationic starch having a solubility of at least 90 percent in water at 65 C. and a viscosity of less than 100 c.p.s. in 5 percent solution in water at 25 C., with about 4 to 6 percent by dry weight of an alkali metal phosphate or mixture of alkali metal phosphates having a pH near neutrality.

2. A composition according to claim 1, said ketene dimer being a compound or a mixture of compounds having the general formula [RCH==C=0] wherein R is an alkyl radical having at least eight carbon atoms, a cycloalkyl radical having at least six carbon atoms, or a phenyl, naphthylor benzyl radical.

3. A composition according to claim 1, said dextrin phosphate having about 0.80 to 1.10 percent by weight of hosphorus molecularly bound therein and also containing about 1 to 3 percent by weight of phosphorus in phosphorus compounds not molecularly bound therein.

4. A composition according to claim 1 said admixture containing by dry weight at least 70 percent of said dextrin phosphate and about 2 to percent of said ketene dimer.

5. A composition according to claim I, said admixture also containing a nonionic emulsifier for said ketene dimer in an amount between 10 and 30 percent of the weight of said ketene dimer.

6. A composition according to claim 1, said admixture also containing between 0.5 and 10 percent by dry weight of a substance that forms a complex with aluminum ions, selected from the group consisting of alkali metal fluorides, nitrilo-triacetic acid, ethylenediamine tetra-acetic acid, tartaric acid, citric acid, phosphoric acid and water soluble salts of said acids.

7. A composition according to claim 1, said admixture also containing between 2 and 10 percent by dry weight of an alkali metal carbonate.

8. A composition according to claim 1, said admixture also containing up to percent by dry weight of a cold water soluble polysaccharide selected from the group consisting of car- 0 web of matted fibers, which comprises dispersing in water a composition according to claim 1 in finely divided solid form, with the water at a temperature below the decomposition temperature of said ketene dimer, and applying the resulting dis ersion to the surface of the web.

1. Method according to claim 10, said composition containing a nonionic emulsifier for the ketene dimer and being dispersed by stirring it into water at a temperature between 50 and C.

12. Method according to claim 10, said composition being dispersed in an amount providing a dry substance content of between 2 and 8 percent by weight in said dispersion.

13. Method according to claim 10, wherein a finely divided solid composition containing by dry weight at least 70 percent of said dextrin phosphate, about 4 to 8 percent of higher ketene dimer mixed with a nonionic emulsifier therefor, about 5 to 10 percent of guar, about 2 to 5 percent of sodium fluoride and about 3 to 5 percent of alkali metal carbonate is stirred into water at a temperature between 50 and 80 C. so as to provide a dry substance content of between 2 and 8 percent by weight in the resulting dispersion.

14. A paper, cardboard or like web of matted fibers having its surface sized with a composition according to claim 1.

15. A paper, cardboard or like web of matted fibers having its surface sized with a composition according to claim 1 1.

16. A composition for surface sizing paper, cardboard or a like web of matted fibers, consisting essentially of a finely divided admixture of, by dry weight, about 2 to 15 percent of a higher ketene dimer and an emulsifier for said ketene dimer, at least 70 percent of a cold water soluble dextrin phosphate consisting essentially of a thermally degraded phosphorylated starch having between 0.3 and 3 percent of phosphorus molecularly bound therein and also containing about 1 to 3 percent of phosphorus in phosphorus compounds not molecularly bound therein, and having a light color, a solubility higher than 30 percent in water at 25 C. and higher than 75 percent in water at 65 C. and a viscosity below 500 c.p.s. in 5 percent solution in water at 25 C., between 0.5 and 10 percent of alkali metal fluoride, between 2 and 10 percent of alkali metal carbonate, and about 5 to 10 percent of a cold water soluble polysacchlaride selected from the group consisting of carboxymethyl, hydroxyethyl and methyl celluloses guar and alginates.

17. The method of surface-sizing paper, cardboard or a like web of matted fibers, which comprises stirring a composition according to claim 18 into water at a temperature between 50 and 80 C., thus forming an aqueous dispersion of said composition, bringing said dispersion to a dry substance content of less than 8 percent by weight, and then applying it to the surface of the web. 

2. A composition according to claim 1, said ketene dimer being a compound or a mixture of compounds having the general formula (R-CH- C- 0)2 wherein R is an alkyl radical having at least eight carbon atoms, a cycloalkyl radical having at least six carbon atoms, or a phenyl, naphthyl or benzyl radical.
 3. A composition according to claim 1, said dextrin phosphate having about 0.80 to 1.10 percent by weight of phosphorus molecularly bound therein and also containing about 1 to 3 percent by weight of phosphorus in phosphorus compounds not molecularly bound therein.
 4. A composition according to claim 1 said admixture containing by dry weight at least 70 percent of said dextrin phosphate and about 2 to 15 percent of said ketene dimer.
 5. A composition according to claim 1, said admixture also containing a nonionic emulsifier for said ketene dimer in an amount between 10 and 30 percent of the weight of said ketene dimer.
 6. A composition according to claim 1, said admixture also containing between 0.5 and 10 percent by dry weight of a substance that forms a complex with aluminum ions, selected from the group consisting of alkali metal fluorides, nitrilo-tri-acetic acid, ethylenediamine tetra-acetic acid, tartaric acid, citric acid, phosphoric acid and water soluble salts of said acids.
 7. A composition according to claim 1, said admixture also containing between 2 and 10 percent by dry weight of an alkali metal carbonate.
 8. A composition according to claim 1, said admixture also containing up to 20 percent by dry weight of a cold water soluble polysaccharide selected from the group consisting of carboxymethyl, hydroxyethyl and methyl celluloses, guar and alginates.
 9. A composition according to said claim 1, said mixture containing by dry weight at least 70 percent of said dextrin phosphate, about 4 to 8 percent of higher ketene dimer mixed with a nonionic emulsifier therefor, about 5 to 10 percent of guar, about 2 to 5 percent of sodium fluoride and about 3 to 5 percent of alkali metal carbonate.
 10. The method of surface sizing paper, cardboard or a like web of matted fibers, which comprises dispersing in water a composition according to claim 1 in finely divided solid form, with the water at a temperature below the decomposition temperature of said ketene dimer, and applying the resulting dispersion to the surface of the web.
 11. Method according to claim 10, said composition containing a nonionic emulsifier for the ketene dimer and being dispersed by stirring it into water at a temperature between 50* and 80* C.
 12. Method according to claim 10, said composition being dispersed in an amount providing a dry substance content of between 2 and 8 percent by weight in said dispersion.
 13. Method according to claim 10, wherein a finely divided solid composition containing by dry weight at least 70 percent of said dextrin phosphate, about 4 to 8 percent of higher ketene dimer mixed with a nonionic emulsifier therefor, about 5 to 10 percent of guar, about 2 to 5 percent of sodium fluoride and about 3 to 5 percent of alkali metal carbonate is stirred into water at a temperature between 50* and 80* C. so as to provide a dry substance content of between 2 and 8 percent by weight in the resulting dispersion.
 14. A paper, cardboard or like web of matted fibers having its surface sized with a composition according to claim
 1. 15. A paper, cardboard or like web of matted fibers having its surface sized with a composition according to claim
 11. 16. A composition for surface sizing paper, cardboard or a like web of matted fibers, consisting essentially of a finely divided admixture of, by dry weight, about 2 to 15 percent of a higher ketene dimer and an emulsifier for said ketene dimer, at least 70 percent of a cold water soluble dextrin phosphate consisting essentially of a thermally degraded phosphorylated starch having between 0.3 and 3 percent of phosphorus molecularly bound therein and also containing about 1 to 3 percent of phosphorus in phosphorus compounds not molecularly bound therein, and having a light color, a solubility higher than 30 percent in water at 25* C. and higher than 75 percent in water at 65* C. and a viscosity below 500 c.p.s. in 5 percent solution in water at 25* C., between 0.5 and 10 percent of alkali metal fluoride, between 2 and 10 percent of alkali metal carbonate, and about 5 to 10 percent of a cold water soluble polysacchlaride selected from the group consisting of carboxymethyl, hydroxyethyl and methyl celluloses guar and alginates.
 17. The method of surface-sizing paper, cardboard or a like web of matted fibers, which comprises stirring a composition according to claim 18 into water at a temperature between 50* and 80* C., thus forming an aqueous dispersion of said composition, bringing said dispersion to a dry substance content of less than 8 percent by weight, and then applying it to the surface of the web. 